Figure 7.

Mutations to the conserved salt-bridge network perturb voltage dependence of metal transport rate. (A) Average initial metal uptake rates ± SEM (n ≥ 4) for DraNramp mutants at different ΔΨ (0 to −120 mV, colored bars). [M²⁺] was 750 µM, and the pH was 7 on both sides of the membrane. The fold increase in transport rate is indicated (cyan, 0 to −120 mV for Cd²⁺; pink, −40 to −120 mV for Mn²⁺). Mutations to E124, D131, E134, R352, and R353 (red) reduced the ΔΨ dependence of Cd²⁺ and Mn²⁺ transport rates across the range of tested voltages, such that they exceeded WT at low-magnitude ΔΨ but lagged WT at physiological large-magnitude ΔΨ. Mutations to N59, M230, H232, and H237 (black) retained WT-like ΔΨ dependence, and D56 mutants (gray) eliminated all metal transport. (B) Network schematic illustrates clustering of residues most important for ΔΨ dependence (red). See also Fig. S5 for representative time traces of these results and Fig. S6 for dependence of in vivo Co²⁺ uptake rates on external [K⁺].